Convert CONFIG_SPL_STACK to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_STACK

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/i12-tvbox_defconfig b/configs/i12-tvbox_defconfig
index a336dd4..29cea18 100644
--- a/configs/i12-tvbox_defconfig
+++ b/configs/i12-tvbox_defconfig
@@ -7,6 +7,7 @@
 CONFIG_MACPWR="PH21"
 CONFIG_VIDEO_COMPOSITE=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SPL_STACK=0x8000
 CONFIG_SPL_I2C=y
 CONFIG_SYS_PBSIZE=1024
 CONFIG_SYS_I2C_MVTWSI=y